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Scientists show they can halt mental decline by cleaning brain of sticky plaque.

The first drug to combat Alzheimer's disease is on the horizon after scientists proved they can halt mental decline by clearing the sticky plaques from the brain that cause dementia.

The breakthrough was hailed as the "best news" in dementia research for 25 years and a potential "game changer" for people with the condition.

Scientists were amazed to find that patients treated with the highest dose of the antibody drug aducanumab experienced an almost complete clearance of the amyloid plaques that prevent brain cells communicating and lead to irreversible memory loss and cognitive decline.

Crucially, they also found that after six months into the treatment, patients stopped deteriorating, in contrast to those taking placebos, suggesting their dementia had been halted.

If shown to be effective in larger trials, the first drug to prevent dementia could be available in a few years.

"The results of this clinical study make us optimistic that we can potentially make a great step forward in treating Alzheimer's disease," said Prof Roger Nitsch of the Institute for Regenerative Medicine at the University of Zurich. "In the high-dose group, the amyloid has almost completely disappeared. The effect size of this drug is unprecedented. Despite it being a small sample, there appeared to be a slowing of cognitive decline and functional decline. The group with a high degree of amyloid removal were basically stable. If we could reproduce this, it would be terrific."

Dr Alfred Sandrock, from the Massachusetts-based biotech company Biogen, which is hoping to bring the drug to market, said: "This is the best news that we have had in our 25 years, and it brings new hope to patients with this disease."

There are currently 850,000 people living with dementia in Britain which is due to rise to one million by 2025 and two million by 2050. The most common kind of dementia is Alzheimer's disease, but until now scientists have argued over the cause and despite more than 400 drug trials, nothing has been shown to combat the disease.

The last Alzheimer's drug licensed in the UK became available more than a decade ago. Current treatments can reduce symptoms to some extent, but doctors have nothing that can halt or slow progression of the illness.

Not only does the new study suggest a treatment for the disease, but shows that the build-up of amyloid plaque in the brain is likely to be to blame.

Aducanumab is a treatment made up of antibodies, tiny Y-shaped proteins that latch on to dangerous substances in the body like flags, showing the immune system what to clear away.

Scientists tested various human immune cells with amyloid in a lab until they found one that produced an antibody which broke up the plaques. They then cloned it in large numbers for the new therapy, which is given intravenously once a month.

In the trial, which was reported in the journal Nature, scientists tested varying levels of the drug over a year, as well as giving one group a placebo. They found that more amyloid was removed as the dose increased, which suggests it was having an impact. Brain scans of those given the highest dose shown virtually no amyloid left at all.

The drug is likely to be most effective for patients in the very earliest stages of Alzheimer's disease, or those who have not yet begun to show symptoms. Several universities are working on early blood tests for dementia that could pick the disease up a decade or more before the first physical signs appear.

Dementia experts and charities said that the breakthrough offered "real hope" for the treatment of the disease.